1. Field of the Invention
The present invention relates to antifuses for use in programmable logic devices. More particularly, the present invention relates to amorphous silicon antifuses having at least one oxide layer.
2. Description of Related Art
Programmable logic devices such as field programmable gate arrays generally include a plurality of programmable logic blocks and a programmable interconnect structure for interconnecting the logic blocks. In one implementation, shortable elements, called antifuses, are incorporated into the programmable interconnect structure to form programmable connections between the logic blocks.
Antifuses are programmable elements which before programming have a high impedance and which upon the application of a sufficient voltage, referred to herein as a programming voltage, change to a low impedance conductive state. Antifuses are generally formed of two conductive layers which serve as electrical terminals and which are separated by an antifuse layer whose resistance is changed by the application of the programming voltage across the terminals. As used herein, an unprogrammed antifuse refers to an antifuse in a high impedance state and a programmed antifuse refers to an antifuse which has been exposed to a programming voltage to reduce the impedance of the antifuse.
Antifuses may be generally divided into two categories, dielectric and amorphous silicon antifuses. Dielectric antifuses generally consist of at least one dielectric layer positioned between two conductive layers, most commonly N+ diffusion and polysilicon. Upon the application of a programming voltage, the dielectric is broken down such that the antifuse becomes conductive. Chiang, et al., "Antifuse Structure Comparison for Field Programmable Gate Arrays" IEDM IEEE pp. 611-614 (1992) discuss a number of dielectric antifuse structures.
Amorphous silicon antifuses generally consist of an antifuse layer formed of amorphous silicon (a-Si, Si.sub.x H.sub.y N.sub.z) positioned between two conductive layers, most commonly two metal layers. When a programming voltage is passed through the amorphous silicon layer, the amorphous silicon undergoes a phase change to a lower impedance state. U.S. Pat. No. 5,181,096 to Forouhi discloses an antifuse in which the dielectric layer includes an amorphous silicon (a-Si) layer positioned between two dielectric films. The dielectric films are formed by either a low pressure chemical vapor deposition (LPCVD) or plasma enhanced CVD (PECVD).
Several performance characteristics are used to evaluate the performance of an antifuse and its usefulness as a programmable element in a programmable interconnect structure. These performance characteristics include leakage current, programming voltage, post programming resistance, time dependent dielectric breakdown and defect failure lifetime.
Leakage current refers to the amount of current which passes through an antifuse in its unprogrammed state at a given voltage. The leakage current of a particular antifuse can vary with temperature. Since the leakage current can cause errors in data transmission, it is preferred that the leakage current of an unprogrammed antifuse be as low as possible. The leakage current of an antifuse is dependent on the quality of the antifuse layer as well as the thickness of the antifuse layer. For example, leakage current can be reduced by increasing the antifuse layer's thickness. However, the programming voltage of the antifuse increases as the antifuse layer's thickness increases.
The programming voltage of an antifuse must be higher than the operating voltage (Vcc) of the programmable logic device so that the antifuse is not programmed during the normal operation of the device. As lower operating voltages are used, the need for antifuses which can operate at lower programming voltages will increase. Since the amount of voltage needed to program an antifuse depends on the thickness of the antifuse layer used, a need exists for antifuses having thinner antifuse layers.
Prior to the antifuse being programmed, the antifuse acts as an off device since the antifuse prevents current from passing through the antifuse. After programming, the antifuse forms a linkage through which electrical signals are conveyed. In order for the device to operate at high speeds, the resistance of the antifuse after programming should be as low as possible. Antifuses having a low resistance after being programmed have been found to be more reliable than antifuses having a higher programmed resistance. Although the mechanism for device failure is not well understood, the heat generated within the device due to the resistance of the programmed antifuse is believed to contribute to device failure. The resistance of the programmed antifuse is generally proportional to the thickness of the antifuse layer used in the antifuse structure. It is therefore preferred that the antifuse layer be as thin as possible.
The time dependent dielectric breakdown (TDDB) of an antifuse refers to breakdown of the antifuse over time due to the antifuse being exposed to the operating voltage (Vcc). Since the TDDB of an antifuse is directly related to the long term reliability of the antifuse, it is preferred that the TDDB of the antifuse be as long as possible.
The defect failure lifetime of an antifuse relates to the time dependent frequency with which antifuses have or develop defects which cause unprogrammed antifuses to be conductive. Since the defect failure lifetime of an antifuse is directly related to the long term reliability of the antifuse, it is preferred that the defect failure lifetime of the antifuse be as long as possible.
As discussed above, increasing the thickness of the antifuse layer advantageously reduces the leakage current of the antifuse and reduces the frequency of antifuse breakdown due to defects in the dielectric layer or the application of a voltage across the antifuse. However, increasing the thickness of the antifuse layer has the undesirable effects of increasing the programming voltage of the antifuse as well as the resistance of the programmed antifuse layer.
A need therefore exists for an antifuse having a thin antifuse layer which provides the combined advantages of having a low current leakage and a low frequency of antifuse breakdown due to defects in the dielectric layer or the application of a voltage across the antifuse, while providing a low programming voltage and a low resistance once programmed.